1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device having a package and a semiconductor chip mounted therein.
2. Description of the Related Art
FIGS. 1A, 1B and 1C show a conventional semiconductor device of this sort. This device is a solid state imaging module 80 and used for a facsimile, an electronic board apparatus and the like. In FIG. 1B, there are shown a relative positional relationship among an optical system 60, a rod lens 63 and the module 80 in a facsimile as well as a cross section of the module 80 taken along an X--X' line in FIG. 1A.
The solid state imaging module 80 has a semiconductor chip of a solid state imaging device 51 housed in a package. The device 51 is a strip-like one-dimensional image sensor with a size of 80 mm.times.1 mm. The package is composed of a body 52, and a transparent cap 53 which is formed by a rectangular plate and fixed on an opening end of the body 52. The body 52 of the package has a rectangular insulating ceramic substrate 70 and rectangular frame-like insulating ceramic plates 71 and 72. The device 51 is mounted on an upper face of the substrate 70 and at the center thereof. The ceramic plate 71 is fixed on the periphery of the upper face of the substrate 70, with surrounding the device 51. The ceramic plate 72 is fixed on the plate 71. A cavity is formed on the substrate 70 by the plates 71 and 72. The solid state imaging device 51 is housed in the cavity. The bottom face of the device 51 is adhered on the upper face of the substrate 70 by an adhesive material of synthetic resin containing silver powder.
The rectangular cap 53 is made of a transparent material such as glass, plastics or the like, and adhered on the ceramic plate 72 by a sealing material 54 in order to seal the cavity for protecting the device 51. The sealing material 54 is either an adhesive made of synthetic resin or low-melting point glass. The ceramic substrate 70 constitutes the bottom of the package, the cap 53 constitutes the top of the package and the ceramic plates 71 and 72 constitute the side wall of the package.
Inner leads 57a, each of which is made of an electroconductive film, are disposed on the upper face of the ceramic plate 71 perpendicular to the longitudinal axis of the plate 71. Parts of the inner leads 57a are disposed between the plates 71 and 72. On the upper face of the solid state imaging device 51, electrodes or bonding pads (not shown) are formed at positions corresponding to the respective inner leads 57a. As shown in FIG. 1A, the bonding pads and the inner leads 57a are interconnected with each other by metal wires 55 which are attached by the wire bonding method. As the wire 55, a gold or an aluminum-silicon alloy wire having a diameter of 30 .mu.m is generally employed.
Outer leads 57 are fixed on the side faces of the body 52 of the package, which are perpendicular to the longitudinal axis of the body 52 and extend up and down. The outer leads 57 are connected to the inner leads 57a respectively.
The thickness of ceramic plate 71 is about the same as the thickness of the solid state imaging device 51. The ceramic plate 72 has a thickness which prevents any problem when forming a loop of the wire 55 during the wire bonding process. A rectangular opening in the ceramic plate 71 is a little larger than the device 51 so that the device 51 is not in contact with the plate 71. A rectangular opening in the ceramic plate 72 is a little larger than the opening in the plate 71 to prevent any problem during the bonding of the bonding pads of the device 51 to the inner leads 57a by the wires 55 in the wire bonding process.
In case that images wider than the width of the imaging module 80 are processed, a plurality of the modules 80 are adjacently disposed linearly. The respective images are simultaneously processed by the plurality of modules 80. It is necessary to reduce the deviation and distortion of the images generated at the boundaries of the plurality of modules 80 in order to proceed the images precisely. Thus, each of the modules 80 should have a width which is as wide as possible.
However, when each of the modules 80 has such a wider width, there is a problem because the flatness of the mounting face or the upper face of the ceramic substrate 70, on which the solid state imaging device 51 is mounted, is reduced the surface-roughness or irregularity of the mounting face is increased. For example, when the surface-roughness of the upper face of the substrate 70 is measured from one end of the face to the other by a surface-roughness tester, the maximum warpage of .+-.150 .mu.m may be obtained.
Here, the "warpage" 61 of the mounting face means a distance between the center of the upper face (mounting face) and the end of the area contacting the device 51 in the direction perpendicular to a line connecting the both ends of the contacting area. Since the warpage of the substrate 70 is on the order of microns, it is invisible to the naked eye. Thus, the warpage is enlarged to be shown in FIG. 1B to make it clear.
For example, in a facsimile, as shown in FIGS.. 1B and 1C, beams 62 of light emitted from an optical system 60 are respectively focused by a rod lens 63 to be irradiated on a light-receiving face of the solid state imaging device 51. In order to obtain clear images, it is necessary for the light-receiving face to be put in a region which is 20 .mu.m or less away from the focal point 64 of the beams 62 conventionally, it is necessary to select part of the fabricated package body 52, which have a warpage that is enough to obtain clear images.
As described above, with the conventional solid state imaging module 80, it is not possible to use all of fabricated package body 52, so that the cost of the package becomes very high.